How does an AEC system determine exposure and what is the role of ionization chambers?

• A. Ionization chambers sense transmitted radiation; exposure ends when the chamber reaches the preset charge corresponding to desired image density.
• B. Ionization chambers generate electrons to control exposure.
• C. AEC adjusts mA in real-time to maintain exposure.
• D. Ionization chambers regulate filtration.

Answer: (A)

Automatic exposure control works by measuring how much radiation actually reaches a sensor and using that to stop the exposure at the right moment. Ionization chambers are the sensing elements in this system. They are gas-filled chambers placed in the beam path (often under the patient or near the detector). As x-rays pass through the patient, they ionize the gas inside the chamber, producing ions. An electric field collects these ions, creating a current that is proportional to the amount of transmitted radiation. The AEC circuit integrates this signal, and when the accumulated charge reaches the preset value that corresponds to the desired image density, the exposure is terminated. This allows for consistent image density across different patient sizes and positions. If the sensing becomes unreliable or the exposure risk is high, a back-up timer will stop the exposure to prevent overexposure.